What happens if a computer has a virus that cant be deleted?

It depends on what kind of virus the computer is infected.

Adware, Backdoor, Dialler, Hijackers, Spyware ...

Then the computer is broken. But what you can do is wipe everything on it and that kills the virus. If it's a blue screen of death then you need a computer genius to type some code and delete the blue screen of death. However if it is a Trojan virus there is no way of get tut rid of it